1. Field of Invention
The present invention relates to light emitting diodes, more particularly to light emitting diodes with a photonic crystal structure.
2. Description of Related Art
Light emitting diodes (“LEDs”) are technologically and economically advantageous solid state light sources. LEDs are capable of reliably providing light with high brightness, hence in the past decades they have come to play a critical role in numerous applications, including flat-panel displays, traffic lights, and optical communications. An LED includes a forward biased p-n junction. When driven by a current, electrons and holes are injected into the junction region, where they recombine, releasing their energy by emitting photons. The quality of an LED can be characterized, for example, by its extraction efficiency that measures the intensity of the emitted light for a given number of photons generated within the LED chip. The extraction efficiency is limited, among others, by the emitted photons suffering multiple total internal reflections at the walls of the high refractive index semiconductor medium. As a result, the emitted photons do not escape into free space, leading to poor extraction efficiencies, typically less than 30%.
In the past thirty years, various approaches have been proposed to enhance the extraction efficiency of LEDs. The extraction efficiency can be increased, for example, by enlarging the spatial angle in which the emitted photons can escape by developing suitable geometries, including cubic, cylindrical, pyramidal, and dome like shapes. However, none of these geometries can entirely eliminate losses from total reflection.
A further source of loss is the reflection caused by the refractive index mismatch between the LED and the surrounding media. While such losses could be reduced with an anti-reflection coating, complete cancellation of reflection can be achieved only at a specific photon energy and one angle of incidence.
U.S. Pat. No. 5,955,749, entitled “Light Emitting Device Utilizing a Periodic Dielectric Structure,” granted to J. Joannopoulos et al., describes an approach to the problem of enhancing the extraction efficiency. According to U.S. Pat. No. 5,955,749 a photonic crystal is created by forming a lattice of holes in the semiconductor layers of the light emitting diode. The lattice of holes creates a medium with a periodically modulated dielectric constant, affecting the way light propagates through the medium. The photons of the light emitting diode can be characterized by their spectrum or dispersion relation, describing the relation between the energy and the wavelength of the photons. The spectrum of a photonic crystal consists of two classes. Photons in the radiative class have energies and wavelengths that match the spectrum of photons in free space thus the radiative photons are capable of escaping from the light emitting diode. Photons in the guided class, on the other hand, have energies and wavelengths that do not match the spectrum of photons in free space; therefore, guided photons are trapped in the light emitting diode. The guided photons are analogous to the earlier described photons, suffering total internal reflections.
The spectrum of guided photons in the photonic crystal consists of energy bands, or photonic bands, separated by band gaps, in analogy with the spectrum of electrons in crystalline lattices. Guided photons with energies in the band gap cannot propagate in the photonic crystal. In contrast, the spectrum of the radiative photons is a continuum, and thus has no gap. The recombinative processes in a typical LED emit photons with a well-defined energy. If, therefore, a photonic crystal is formed in the LED such that the energy of the emitted photons falls within the band gap of the photonic crystal, then all the emitted photons are emitted as radiative photons as no guided photons can exist with such energies. As described above, since all the radiative photons are capable of escaping from the LED, this design increases the extraction efficiency of the LED.
In an effort to explore the usefulness of photonic crystals for light generation, U.S. Pat. No. 5,955,749 gives a partial description of a theoretical structure of a photonic crystal device.
U.S. Pat. No. 5,955,749 describes an n-doped layer, an active layer, and a p-doped layer, and a lattice of holes formed in these layers. However, the device of U.S. Pat. No. 5,955,749 is not operational and therefore is not a LED. First, electrodes are not described, even though those are needed for the successful operation of a photonic crystal LED (“PXLED”). The fabrication of electrodes in regular LEDs is known in the art. However, for PXLEDs, neither the fabrication of electrodes, nor their influence on the operation of the PXLED is obvious. For example, suitably aligning the mask of the electrode layer with the lattice of holes may require new fabrication techniques. Also, electrodes are typically thought to reduce the extraction efficiency as they reflect a portion of the emitted photons back into the LED, and absorb another portion of the emitted light.
Second, U.S. Pat. No. 5,955,749 proposes fabricating photonic crystal light emitting devices from GaAs. GaAs is indeed a convenient and hence popular material to fabricate regular LEDs. However, it has a high “surface recombination velocity” of about 106 cm/sec as described, for example, by S. Tiwari in “Compound Semiconductor Devices Physics,” Academic Press (1992). The surface recombination velocity expresses the rate of the recombination of electrons and holes on the surface of the diode. Electrons and holes are present in the junction region of the LED, coming from the n-doped layer and the p-doped layer, respectively. When electrons and holes recombine across the semiconductor gap, the recombination energy is emitted in the form of photons and generates light. However, when electrons and holes recombine through intermediate electronic states in the gap, then the recombination energy is emitted in the form of heat instead of photons, reducing the light emission efficiency of the LED. In an ideal crystal there are no states in the gap. Also, in today's high purity semiconductor crystals there are very few states in the gap in the bulk material. However, on the surface of semiconductors typically there are a large number of surface states and defect states, many of them in the gap. Therefore, a large fraction of electrons and holes that are close to the surface will recombine through these surface and defect states. This surface recombination generates heat instead of light, considerably reducing the efficiency of the LED.
This problem does not result in a serious loss of efficiency for regular LED structures. However, PXLEDs include a large number of holes, thus PXLEDs have a much larger surface area than the regular LEDs. Therefore, the surface recombination may be capable of reducing the efficiency of the PXLED below the efficiency of the same LED without the photonic crystal structure, making the formation of photonic crystal structure pointless. Since GaAs has a high surface recombination velocity, it is not a promising candidate for fabricating photonic crystal LEDs. The seriousness of the problem is reflected by the fact that so far, to Applicants' knowledge, no operating LED with a photonic crystal near the active region has been reported in the literature that uses GaAs and claims an enhanced extraction, or internal, efficiency. In particular, U.S. Pat. No. 5,955,749 does not describe the successful operation of a photonic crystal LED. Also, U.S. Pat. No. 5,955,749 does not describe the influence of the photonic crystal on the emission process, which can affect the internal efficiency of the LED.
While photonic crystals are promising for light extraction for the reasons described above, there are problems with the design. There are several publications describing experiments on a lattice of holes having been formed in a slab of a semiconductor. An enhancement of the extraction rate at photon energies in the bandgap has been reported by R. K. Lee et al. in “Modified Spontaneous Emission From a Two-dimensional Photonic Bandgap Crystal Slab,” in the Journal of the Optical Society of America B, vol. 17, page 1438 (2000). Lee et al. not only shows the extraction benefits of a photonic crystal in a light emitting design, but also shows that the photonic lattice can influence the spontaneous emission. However, Lee et al. do not show how to form and operate a light emitting device with this design. A photonic crystal LED can be formed from Lee et al.'s light emitting design by including electrodes. The addition of the electrodes, however, will substantially affect the extraction and the spontaneous emission. Since this effect is unknown, it cannot be disregarded in the design of a LED. Since the Lee et al. design does not include such electrodes, the overall characteristics of an LED, formed from that design, are unclear. This questions the usefulness of the design of Lee et al.
Therefore, there is a need for new designs to create operational photonic crystal LEDs. This need includes the introduction of new materials that have sufficiently low surface recombination velocities. The need also extends to designs that counteract predicted negative effects, such as reduced spontaneous emission rates and reflection by electrodes. Finally, there is a need for describing techniques for the fabrication of photonic crystal LEDs, including fabricating electrodes.